Thanksgiving Skies

November 22, 2005: Thanksgiving is the biggest travel
holiday of the year in the United States. Millions of people
board airplanes and fly long hours to visit friends and family.

Do
you dread the trip? Think of it as a sky watching opportunity.
There are some things you can see only through the window
of an airplane. Atmospheric optics expert Les Cowley lists
a few of his favorites:

"Both
sides of the aircraft have their own sights," says Cowley.
"On the side opposite the sun, the main thing to look
for is the glory. Clouds below the aircraft are required.
They are the canvas on which the glory is 'painted.'"

"Look
toward the antisolar point, the place in the clouds directly
opposite the sun," he instructs. "There, if the
aircraft is low enough, you will find the shadow of the plane.
Surrounding the shadow is the glory--a bright white glow surrounded
by one or more shimmering rings of color."

"These
rings are formed when light is scattered backwards by individual
water droplets in the cloud.
The more uniform the size of the cloud droplets, the more
rings you will see. They swell and contract as you travel
over clouds with smaller or larger droplets."

"In
that case," says Cowley, "another optical effect
might be visible, especially over arid regions or pine forests.
This is the opposition effect, a bright patch of
light moving along the ground below you. The brightening,
which is always directly opposite the sun, marks the point
where the shadows of objects, like trees or soil granules,
are hidden beneath those objects. The area consequently looks
brighter, and slightly more yellow, than the surroundings."
(Click
here to view an image of the opposition effect, photographed
by Eva Seidenfaden flying over Uzbekstan.)

Turning
to the sunward side of the aircraft...

"That
is the realm of ice halos," says Cowley. Ice halos are
rings and arcs of light caused by ice crystals in high clouds.
"They are often rainbow-colored," he notes, "but
they are not rainbows."

From
the ground you look up to see these halos. From an
airplane you look down.

"You
might be able to see subhorizon halos invisible from
low ground," says Cowley. "The brightest, sometimes
blindingly bright, is the subsun. This is a direct reflection
of the sun from millions of flat plate-shaped ice crystals
floating in the clouds beneath you and acting together as
a giant mirror. As the aircraft moves the subsun drifts along
the clouds, sometimes growing, sometimes contracting, sometimes
wobbling as crystals with different tilts are sampled. Sometimes
a column of light will extend upward from the subsun toward
the real sun--this is a lower sun pillar."

"Sunrise
and sunset from high altitudes are special," Cowley adds.
"The speed of the aircraft can make them faster or slower
than usual. Furthermore, the sun is extra-flattened because
its light is refracted almost twice the normal amount by its
passage into the dense lower atmosphere and then out again
to you. On a night flight, you might catch the moonrise; its
distortions and flattening are greater for the same reason."

"And
if none of these things are visible on your particular flight,
ignore fellow passengers and crane your head to see some of
the sky above you. It is dark and a deep violet blue--darker
than you will ever see on the ground. A large part of Earth's
atmosphere is beneath and there are far fewer molecules to
scatter the sun's light and turn the sky blue. You are not
far from space."